In the formula, R.sup.10 is a hydrogen atom or an alkyl group having from 1 to 10 carbon atoms, R.sup.11 and R.sup.12 are hydrogen atoms or the same or different protective groups, respectively, and R.sup.5 ' is a straight chained or branched alkyl group.
In the above formula (III'), numerals 1 to 15 alloted to the carbon atoms, designate the positions of the carbon atoms.
Natural prostaglandin I.sub.2 (hereinafter referred to as "PGI.sub.2 " including synthesized products) is chemically unstable, and it is difficult to handle it in a neutral or weakly acidic condition. Whereas, 7-fluoroprostaglandins I.sub.2 represented by the above formula (III') are extremely stable under the above condition, and believed to be useful from the aspect of the improvement of the selectivity for pharmacological activities. These 7-fluoroprostaglandins I.sub.2 are prepared by the fluorination of 7-hydroxy PGI.sub.2 (see the above-mentioned Japanese Unexamined Patent Publication No. 165382/1982) or by a process disclosed in the above Japanese Unexamined Patent Publication No. 171988/1982. However, these processes have various problems such as poor yield and selectivity, or the complexity of the reaction steps. Therefore, it has been desired to solve these problems.